(a) Field of the Invention
The invention relates to communication system and method for spread spectrum (SS) communication, and in particular, to communication system and method in which data to be transmitted is modulated according to CSK (code shift keying) technique utilizing a plurality (N) of PN (pseudo-noise) code series.
(b) Description of the Related Ar:
Recently, an SS communication system is widely employed in the fields of base station communication, mobile unit communication, power line communication or the like. A conventional SS communication system will be described with reference to FIGS. 1 and 2. FIG. 1 is a block diagram of a conventional SS communication system including a signal transmitter and a signal receiver, while FIG. 2 is a timing chart showing signal waveforms appearing at specified nodes in the conventional SS communication system shown in FIG. 1.
In FIG. 1, on the signal transmitter side of the system, an output "a" from a PN (pseudo-noise) code series generator 1 and data "b" for transmission are input to an exclusive OR circuit 2, an output "c" of which is amplified by an amplifier 3 to be delivered as a transmitted signal onto a signal transmission path.
On the signal receiver side of the system, the transmitted signal is input to an amplifier 4 as a received signal. An amplified signal which is delivered from the amplifier 4 is input to a synchronized PN code series generator 5 and also to a correlator 6 where the amplified signal is correlated with an output "d" from the synchronized PN code series generator 5. An output signal "e" from the correlator 6 represents a correlation value, which is compared in a comparator 7 against a given threshold value. An output of the comparator 7 is delivered as received data "f".
The signal transmission path may be either a radio path or a wiring, and can be formed by one of a variety of signal transmission media which is adequate to a particular application. The transmitted signal is directly delivered to the signal transmission medium where it is often converted into a signal form which enables an efficient transmission through the signal transmission medium.
In a power line communication, it is necessary to provide an interface which isolates a transmitted signal from commercial electric power. A portion of the system which operates on the signal transmission medium, and which either converts the transmitted signal which passes therethrough into a signal form adequate for the transmission or isolates it from the power line, is referred to as "receiving interface" or "transmitting interface" depending on its use. Such an interface provides a connection means with the signal transmission medium.
In a conventional SS communication system as described above, PN series which is produced by the synchronized PN code series generator 5 on the signal receiver side must be synchronized with the PN series which is delivered from the transmitter side. At this end, it is essential to seek for a point of synchronization therebetween.
If the signal transmission path is satisfactory in respect of the signal transmission characteristic, a correlation waveform will exhibit a peak at the synchronized point, as illustrated in FIG. 3A, presenting no problem whatsoever. However, when there is a dip point within the signal transmission band as when used in the power line communication or when the signal transmission characteristic of the transmission path is considerably low, the correlation waveform will be collapsed as illustrated in FIGS. 3B and 3C. In such an event, the sign of the correlation value will be inverted between "+" and "-", which means that the data level will be inverted between "1" and "0" and the synchronization can no longer be maintained under the same condition, which involves a serious drawback.
A CSK modulation technique is known to be applicable to SS communication. The CSK modulation techniques applied to SS communication system include ones disclosed, for example, in JP-A-2(1990)-246539 and JP-A-2(1990)-6540.about.6548.